Epoxides such as ethylene oxide, propylene oxide, 1,2-butene oxide and the like are useful intermediates for the preparation of a wide variety of products. The oxirane functionality in such compounds is highly reactive and may be ring-opened with any number of nucleophilic reactants. For example, epoxides may be hydrolyzed to yield glycols useful as anti-freeze components, food additives, or reactive monomers for the preparation of condensation polymers such as polyesters.
Polyether polyols generated by the ring-opening polymerization of epoxides are widely utilized as intermediates in the preparation of polyurethane foams, elastomers, sealants, coatings, and the like. The reaction of epoxides with alcohols provides glycol ethers, which may be used as polar solvents in a number of applications.
Many different methods for the preparation of epoxides have been developed. One such method involves the use of certain organorhenium oxide compounds to catalyze olefin oxidation by hydrogen peroxide. This method is described, for example, in German Patent No. 3,902,357 and Herrmann, J. Organomet. Chem. 382, 1(1990). While high yields of certain epoxides may be obtained by this procedure, attempts to prepare other epoxides were much less successful. In particular, these publications teach that a 1,2-diol by-product is often produced in addition to or instead of the desired epoxide. The formation of such by-products is especially favored, according to the prior art, when the reaction temperature exceeds 10.degree. C. Maintaining an epoxidation reaction mixture below 10.degree. C. will be impractical on a commercial scale owing to the special cooling equipment required and the high utility costs associated with rapidly removing heat from an exothermic reaction of this type. It would be highly desirable to develop an epoxidation process using hydrogen peroxide oxidant and organorhenium oxide catalyst which could be effectively operated at a temperature above 10.degree. C. so as to give a product which is exclusively epoxide.
The prior art additionally teaches that it is beneficial to employ a hydrogen peroxide solution that does not contain any water and recommends the use of an organic solvent as a liquid medium for the epoxidation reaction. Suitable solvents are said to include tetrahydrofuran, monovalent aliphatic alcohols with 1-5 carbon atoms, and aromatic hydrocarbons such as toluene and xylene. Solutions in tert-butanol are taught to be especially preferred. However, hydrogen peroxide is currently available commercially only in the form of aqueous solutions. To employ one of the organic solvents recommended by the prior art, it will thus be necessary to exchange the water of a typical hydrogen peroxide solution for the organic solvent. This will necessarily increase greatly the overall costs associated with an epoxidation process of this type. Additionally, concentration of hydrogen peroxide to a pure or nearly pure state is exceedingly dangerous and is normally avoided. Thus, it will not be practicable to simply remove the water by distillation and replace it with the organic solvent. Since hydrogen peroxide has a high solubility in and high affinity for water, liquid-liquid extraction of hydrogen peroxide from an aqueous phase to an organic phase will not be feasible. Moreover, certain of the solvents taught by the prior art to be preferred for epoxidation reactions of this type such as tert-butanol are water miscible and thus could not be used in such an extraction scheme. An epoxidation process wherein a readily obtained oxidant solution is employed containing hydrogen peroxide and an organic solvent which promotes high yields of epoxide products would thus be of significant economic advantage.